The millimeter-wave spectrum of methyltrichlorosilane

JOURNAL

OF MOLECULAR

SPECTROSCOPY

The Millimeter-Wave

136, 109-119 (1989)

Spectrum of Methyltrichlorosilane JOHN

G. SMITH

Department of Chemistry. The University of Newcastle upon Tvne. Newcastle upon Tyne, NE1 7RU. United Kingdom The millimeter-wave rotational spectra of CHJ SC& have been recorded and analyzed for both the ground and the two lowest degenerate fundamental vibrational states. The quadrupole splitting due to the presence of three 3sC1nuclei has been analyzed for the ground state. For the two excited states, the I-resonance has been analyzed and values of the Coriolis constants have been determined. The C rotational constant has been determined for one of these states. o 1989 Academic press, hc. INTRODUCTION

We have previously investigated the rotational spectra of several trichloro species in this laboratory ( 1,2). Such investigations have led to information on both the force fields of these molecules (3) and the nature of the quadrupole coupling (4). The latter is in principle extremely complex for a molecule with three identical quadrupolar nuclei. However, at high J the observed splittings are smaller and this leads to some drastic simplifications in the spectra observed with the resolution available in this experiment. Consequently, for many transitions the rotational frequency may be taken to be the center of a multiplet pattern which is often a doublet or a quartet and the analysis can proceed by ignoring the effects of the nuclear quadrupoles. In some special cases the latter may cause unexpected splittings (4) and one must at least be aware that neglect of quadrupolar coupling at high J is an approximation. We present here the investigation of methyltrichlorosilane which is a natural extension of previously published work on trichlorosilane (3, 4). EXPERIMENTAL

DETAILS

All spectra were recorded on the University of Newcastle millimeter-wave spectrometers, which have been described elsewhere (5 ). The absorption cells were cooled to dry ice temperatures for all spectra including the excited vibrational states. Klystrons were used as fundamental sources throughout this work and these were phase-locked to a standard crystal whose frequency is monitored against the 200~kHz BBC Droitwich signal. As a routine check the OCS frequencies reported by Dubrulle et al. (6) were used on a day-to-day basis since the strong OCS signals are regularly used to tune the harmonic generation of millimeter-wave radiation from the klystron sources. Source modulation is employed and all spectra are recorded as second derivatives of the absorption line shape. A commercial sample of CH3SiC13was used throughout this work. Its vapor pressure at dry ice temperatures is near optimum for the modulation depth employed. 109

0022-2852189 $3.00 Copyright 0

1989 by Academic Press. Inc.

All rights of reprmiuctmn in any form reserved.

110

JOHN G. SMITH THEORY

The theory of the hyperfine structure of the rotational spectra of symmetric top molecules with three identical nuclei was first presented by Wolf et al. ( 7). Briefly, the Hamiltonian may be written as H = H,, +

f&ad,

(1)

where ZZ,,, is the normal rotational Hamiltonian employed for a C,, molecule in an A or E vibrational state. References to the theory and its implementation on computer are given in references of earlier work from this laboratory (l-3). The term in Z&a,, gives rise to ( 21 + 1) 3 = 64 spin states. For k # 1, the exact value of these energies depends only on eQqzrwhere z is parallel to the C3, axis, and if the individual electric field gradients are cylindrically symmetrical about the M-Cl bonds then this is equivalent to eQq cos a!where (Yis the Cl-Si-Cl angle and q is directed along the bond. For most molecules that have been studied to date this seems to be a reasonable approximation though there is some evidence in the case of PC13 (8) that the field gradients are not cylindrically symmetrical about the P-Cl bonds. The approach to the calculation used in our own program is different from that presented by Wolf et al. ( 7) and has been reviewed elsewhere (4, 9). As previously mentioned the complexity implied by this theory is not observed due to lack of resolution and the spectra usually show relatively small and simple splitting patterns. Ground State The microwave spectra of CH3SiC13 were first investigated by Mockler et al. ( 10). In this early work they were able to estimate an r. structure for the molecule by making assumptions about the geometry around the carbon. More recently, Mitzlaff et al. ( I I ) recorded and analyzed the ground state spectra of several isotopic species between the frequencies of 17 and 36 GHz. A value was also presented for the centrifugal distortion constant, DJ, which may be compared with the present work in Table II. The problem of the quadrupolar coupling, which severely restricts the accuracy of measurement of these transitions, was not considered. In the present work we have recorded the ground state pure rotational spectrum of CHs28Si35C13from J” = 27 to J” = 59. As mentioned in other papers from this laboratory (4)) the use of millimeter waves results in a great simplification of the spectra. At high .Zthe complexities due to the quadrupolar coupling are not observed at low k values so that an individual k label may be placed on single absorption peaks. At higher k splittings become evident and we have taken the center of such patterns where this is obvious. The exact assignment of the k label was made with the aid of a plot of frequency against k*. The use of the pattern center as the pure rotational transition frequency proved a poor choice for the intermediate .Zvalues due to the magnitude of the centrifugal distortion constant, DJK.Centrifugal distortion splitting of the k transitions is similar in size to the splittings due to the quadrupolar coupling. Simulations of the spectra for these frequencies which included the quadrupole splitting showed quite clearly that there was considerable overlap between one group of AF transitions with the same k and the next k group. This resulted in difficulty in measuring the true quadrupole splitting center and also in very few transitions giving useful information on the magnitude of the quadrupole constant. The observed centers of

MILLIMETER-WAVE

SPECTRUM

OF METHYLTRICHLOROSILANE

111

TABLE I Fit to the Observations J”

K

Ohs

Freq

o-c

Obs

for the Ground Err

J"

State of CH128Si35C1j K

Ohs

Freq

o-c

Obs

Err

0.200 8%% -0:001 0.009 8% 0:058 -0.039

Note. Tr ansitions are J” + 1, K + J”, K. Frequencies error of an obs. of unit weighIt = 0.202 MHz.

in MHz

Number

of obser vations

= 107. Standard

the rotational transitions and their fit is given in Table I. The resulting parameters are presented in Table II. The quadrupole coupling constant has been determined from a few transitions which are not seriously blended. Following the trichlorosilane example (4)) we have simulated the spectra for a given value of eQqZZand compared the center positions of the simulated peaks with those observed in order to refine the quadrupole coupling constant. This fit is given in Table III.

Excited Vibrational States Two excited vibrational states were observed, one on either side of the ground state transitions. Both clearly arise from E species low-lying degenerate vibrational fundamentals. The two lowest such vibrations are essentially the Cl-Si-Cl deformation

112

JOHN

G. SMITH

TABLE11 Ground State Parameters forCH~28Si35Cl~ Value

Std Error

C/MHZ

1317.113

constrained

B/MHz

1769.80237

0.00007

0.30090

0.00003

-0.18520

0.00005

Parameter

DJ/kHz DJK/kRz HJJJ/Hz

0

Ref/ll) 1769.798 0.19

0.04

constrained

HJJK/HZ

-0.00026

eQg/MHz

13.16

0.00006 0.89

oQ

-4.00

constrained

and the rocking of the methyl group with respect to the rest of the molecule, but it has not proved possible to assign our rotational observations to these particular vibrations. In all this work, the center of any observed quadrupole splitting has been taken to be the rotational transition frequency. The Lower Frequency E Excited State To low frequency of the ground state, spectra arising from a degenerate fundamental vibration were observed. The intensity is such that the vibration in question must lie at about 200 cm -’ above the ground state. The pattern consisted of two groups of lines, the upper group being a series which went to low frequency and then formed a band head before turning and giving rise to a weak series of closely spaced high k components. The lower group went from low to high frequency with a much closer spacing than the first and finished by the k transitions blending together. This pattern is typical of that found for Z-resonance in a degenerate vibrational state. The two groups correspond to the two series _+kl - 1. A plot of v’/( 25 + 1) against (J + 1) * ( 1)) where d has been corrected for the effect of DJ such that v’/( 2 J + 1) is the effective TABLE III Fittothe HyperfmeSplittingsin theGround State J"

K

37

29

0.319

-0.002

37

30

0.340

-0.004

37

31

0.377

0.003

37

32

0.397

-0.007

Obs Splitting/MHz

Obs-Calc/MHz

37

33

0.425

-0.004

37

34

0.479

0.021

37

35

0.493

0.017

37

36

0.486

-0.021

37

37

0.522

-0.002

MILLIMETER-WAVE

SPECTRUM

OF METHYLTRICHLOROSILANE

113

B for the transition in question, enabled a straightforward assignment to be made. First, the upper I-doublet was obvious as in this plot it appears as a straight horizontal line. Second, the low values of kl - 1 are easily assigned with the exception that the assignment of which group belongs to which sign of kl - 1 is impossible to decide. In order to resolve this question one would need the evidence of the k = J” transitions or would have to be able to find the magnitude of the quadrupole splitting in the two series. As the high k lines are blended in both series this is not possible. The choice we have made in the fit to the observations presented in Table IV is purely for convenience. If the real sign is the reverse then this corresponds to a change in the sign of the resonance denominator, B - C + Cc, in the I-resonance expression. This results in a rather different value of {but the other parameters are essentially unaltered. The parameters with our choice of assignment are given in Table V. The effects of the Ak = 2 matrix elements which couple the (kl - 1) = + 1, which depend on the parameter VQ, were not observed for this state. The Upper Frequency E Excited State The rotational spectrum of a second excited vibration was found above the ground state transitions. It consisted of a very obvious series of strong lines, the separations of which decreased regularly until the series was lost in a bandhead followed by a complex pattern of weak lines. The same plot as that used in the previous case was tried but there were some obvious differences in the two cases. First, the strong series obviously corresponded to low kl - 1 transitions. However, it is one of the features of this plot that the positive and negative kl - 1 transitions form a series of curves which are symmetrically placed about the hypothetical center of the rotational transitions. In this case there was no transition with the opposite sign of kl - 1 which corresponded to the first member of this strong series. If however the first transition was assigned kl - 1 = +2, rather than the more obvious + 1, then a weak transition in among the complex pattern could be assigned to -2. This posed two questions. First, where was the +l transition and second, why is the -2 so weak and why can we not see an equally obvious negative kl - 1 series? The answer to the first question was readily resolved by a study of the spectra at 99 GHz. Trial refinements to the above assignments followed by predictions determined the region where the f 1 line was expected. A weak very broad feature was found at this frequency. Scans at higher resolution showed this to be composed of at least two transitions. This behavior has been seen in the case of Cl3SiH ( 4). In the case of the transitions with kl - 1 = k 1, there is a matrix element in the quadrupole Hamiltonian which links states which differ by Ak = *2. This element, which depends on a parameter TQ, causes a quadrupole splitting in the kf - 1 = t-1 transitions. The presence of this splitting confu-ms the kl - 1 assignment in the present state. It also answers part of the second question. The reason why it is difficult to observe the - 1 transition is precisely the same. It is broadened by the same term and against a background of high k transitions from both series it is not immediately obvious. The reason that the -2 and -3 transitions are not obvious either is discussed below. Part of the answer again is that they lie on a background of almost continuous absorptions provided by the high k quadrupole components. At this stage, the predictions from initial refinements did indicate that the value of Cc was such that the energies of one component of the -3 and l-doublet A IAZ levels

TABLE IV Fit to the Observations for the Lower Excited Vibrational State J"

kl-1

Sym

Obs Freq

Obs-Calc

Err in Obs

Note. Transitions are J” + 1, kl - 1 + J”, kl - 1. Frequencies in MHz. Number of observations = 9 Standard error of an observation of unit weight = 0.200 MHz.

MILLIMETER-WAVE

SPECTRUM

OF METHYLTRICHLOROSILANE

115

TABLE V Derived Parameters for the Excited Vibrational States Upper

Vib.

State

Value

Parameter

Std Error

'Jib. State

Value

Std Error

l

1324.9053

0.370

1317.113

1770.6040

0.0002

1768.7447

0.0005

0.13

-203.12

0.24

102.58 1.2415

0.001

1.4663

0.4207

0.0011

0

0.0004 * 0.0002

0.29919

0.00008

0.3003

-0.16996

0.00062

-0.1893

0.0017

-0.7220

0.031

-3.32

0.00562 -0.74

*Constrained

Lower

0.00044

0

0.02 *

0.08

0

*

at this value.

were very similar. Such cases have been discussed before ( 12). These two levels are connected in the Hamiltonian by two possible terms: first, ~~~~~~ gives a Ak = 3 matrix element but force field calculations show that the value of this constant is less than 1 1771.227

Q

0

Q

Q

Q

0

Q

8

l

l

l

l

Q

Q

1770.986

1770.745

Q

0

1770.504 l l l

1770.263 l

l

l

1770.072 J

FIG. 1. Plot of Beiragainst (J + 1 )* for the upper vibrational state. The plot is for the two I-doublets and the X-l- 1 = -3 A, A2 pair only. Filled points correspond to observed transitions, open points to calculated values. The values on the X-axis are given in terms of J rather than as (J + 1)*.

116

JOHN

G. SMITH

TABLEVI Fit to the Observations for the Upper Excited Vibrational State J"

kl-1

Sym

Obs Freq

Obs-Calc

Err in Obs

MILLIMETER-WAVE

SPECTRUM

OF METHYLTRICHLOROSILANE

117

TABLE VI-Continued J”

kl-1

Sym

Obs

Freq

Obs-Calc

Err

in

Obs

Note. Transitions are J” + 1, kl - 1 + J”, kl - 1. Frequencies in MHz. Number of observations = 158 Standard error of an observation of unit weight = 0.16 1 MHz.

118

JOHN

G. SMITH

kHz. Second, there is also a possible connection by the so-called 2, -1 Z-resonance term, rl. Since rt is of the same order of magnitude as the I-doubling constant, q:, this provides a substantial degree of coupling, and introduction of this term into our calculations yielded considerable shifts in one of the I-doublet transitions and one component of the Ai& pair that is labeled -3. The reason for our initial failure to observe -3 then becomes clear. If -3 is split then each component is only half the expected intensity and one component at least will be shifted by this resonance. The unshifted component again lies on a background of high

1)* proved invaluable in finding the resonant transitions. Spectra of J values far from the resonant J yielded initial observations on the I-doublet and by a boot-strap process more and more of the resonant transitions could be identified. This plot for the two I-doublets and two components of the -3 A, A2 pair is shown in Fig. 1. The observed resonance also confirms our assignment of which is the positive and which is the negative kl - 1 series. No such interaction between the +3 level and the I-doublet is possible since this would require a Ak = 4 matrix element. The failure to observe the kl - 1 = -2 transition with the expected intensity is more difficult to understand. Undoubtedly part of the answer is the previously mentioned fact that it lies on a background of high k transitions. In favorable cases there was some indication that quadrupolar splittings were observable in this transition. The energy of the -2 level is close to that of the -1 for some J”; e.g., at J” = 29 the separation is some 300 MHz, and these splittings may arise from resonance since these two levels are connected via the 2, - 1 element, r,. A rapid scan of the observed spectrum at J” = 32 is given in Fig. 2 where it is compared to a computer simulation. The final fit to the data is given in Table VI. The parameters used to obtain this fit are in Table V. The axial rotational constant is reasonably well determined by the present data although the correlation with C{ is quite high (91%). As expected, the value determined for r, is of the same order of magnitude as the Z-doubling constant, qt. The distortion constants are very similar to those found for the ground state which is additional evidence that the assignments presented here are correct.

CONCLUSION

The determination of the axial rotational constant yields new structural data for this molecule. We have only determined the value of C for an excited vibrational state but the difference between C, and Co is expected to be comparable with that between B, and &, i.e., no more than 1 MHz. The value obtained, 1325.2 MHz, is close to the value of 13 I7 MHz which may be obtained from the approximate structure of Mockler et al. (20) which lends some support to its reliability. Without further measurements on isotopic species, the data are still not sufficient to define the structure.

MILLIMETER-WAVE

SPECTRUM

OF METHYLTRICHLOROSILANE

119

,

I 16792

116800 0

+1

116810 -3 +2

+3

I 16820 Frequency/MHz

-4

116830 -3

-2

116840 -1

FIG. 2. Low resolution rapid scan of the upper excited vibrational state at J” = 32 compared to a computer simulation.

ACKNOWLEDGMENTS We thank Miss Lucy Clarkson for making some of the measurements and initial assignments on the upper vibrational state and Professor D. H. Whiffen for considerable help with modifications to the quadrupole program.

RECEIVED:

February

21, 1989 REFERENCES

J. H. CARPENTER,R. CRANE, AND J. G. SMITH, J. Mol. Specfrosc. 101,306-318 ( 1983). J. G. SMITH, J. Mol. Spectrosc. 77, 169-177 (1979). J. G. SMITH, J. Mol. Spectrosc. 120, 110-l 17 (1986). J. H. CARPENTERAND J. G. SMITH, J. Mol. Spectrosc. 121,270-277 ( 1987). J. H. CARPENTER,J. D. COOPER,J. B. SIMPSON,J. G. SMITH,AND D. H. WHIFFEN. J. Phys. E 7,678681 (1974). 6. A. DUBRULLE,J. DEMAISON,J. BURIE, AND D. BOUCHER,Z. Natuforsch. A 3547 l-474 ( 1980).

1. 2. 3. 4. 5.

7. A. A. WOLF, Q. WILLIAMS,AND T. L. WEATHERLY, J. Chem. Phys. 47,5 101-5 109 ( 1967). 8. J. H. CARPENTER,A. WALTER, M. RABBETT,AND J. G. BAKER, J. Mol. Spectrosc., in press. 9. 10. Il. 12.

J. H. CARPENTER,P. SEO, AND D. H. WHIFFEN,to be published. R. C. MOCKLER, J. H. BAILEY, AND W. GORDY, J. Chem. Phys. 21, 1710-1715 (1953). M. MITZLAFF, R. HOLM, AND H. HARTMANN, 2. Nuturforsch. A 22, 1415-1418 (1967). R. CRANE AND J. G. SMITH, J. Mol. Spectrosc. 101, 229-244 ( 1983).